Water soluble, biocompatible compounds that impart lubricity to the surface of otherwise non-lubricious materials are desirable for use on medical devices which are inserted or implanted into the body. Such medical devices may include catheters that are utilized to deliver a stent, stent-graft, graft or vena cava filter, balloon catheters, other expandable medical devices and so forth. The industry has turned to hydrophilic lubricious coatings in order to overcome problems with commonly used hydrophobic coatings such as silicone, glycerine or olive oil.
Hydrophobic coatings have been known to bead up and run off when exposed to an aqueous environment, lose initial lubricity rapidly, and lack abrasion resistance. Residual amounts of silicone have also been known to cause tissue reaction and irritation in patients. The loss of lubricity can lead to discomfort during insertion into a patient, and damage to blood vessels and tissues due to frictional forces during insertion or removal of the device.
One class of polymeric substances which dissolve or swell in an aqueous environment, often referred to as "hydrogels," are capable of manifesting lubricity while in a wet state, and are popularly utilized as lubricious coatings for medical devices. When hydrated, these substances have low frictional forces in humoral fluids including saliva, digestive fluids and blood, as well as in saline solution and water. Such substances include polyethylene oxides, optionally linked to the substrate surface by urethane or ureido linkages or interpolymerized with poly(meth)acrylate polymers or copolymers; copolymers of maleic anhydride; (meth)acryl amide polymers and copolymers; (meth)acrylic acid copolymers; poly(vinyl pyrrolidone) and blends or interpolymers with polyurethanes; and polysaccharides.
These water soluble coating materials, while popular because they provide excellent lubricity and biocompatibility, may be sensitive to moisture.
A problem associated with the moisture sensitivity of such hydrogels is that they may prematurely uptake ambient moisture and become sticky or tacky. This can result in undesirable adhesion of the medical device to itself, to other devices if mass packaged, or to any other surface to which it comes in contact during sterilization or storage. In the case of dilatation balloons, after sterilization or storage these hydrogel coatings can become delaminated from the polymeric surface upon expansion of the balloon because the folded sections stick to one another by cross-polymerization or bridging.
In the case of metal wires, such as guide wires, which may be packaged in rolls, the "self adhesive" effect can lead to removal of some of the coating, leaving pinholes or complete failure of the coating from the surface of the wire as it is uncoiled.
These problems are discussed in U.S. Pat. No. 5,509,899 issued Apr. 23, 1996 to Fan et al. Fan et al. teaches a medical balloon and catheter which is wrapped and folded upon itself and in which the balloon is free of bridging and adhesion between abutting surfaces. The balloon has a base of a continuous polymeric surface which is expandable. On the polymeric surface is disposed a lubricious, biocompatible hydrogel coating and a thin, lubricious, blood-compatible coating is disposed on the hydrogel coating and adheres to it to prevent abutting surfaces of the folded polymeric surfaces from adhering to each other during inflation and to prevent delamination of the hydrogel coating and/or rupture of the balloon. Preferably, the blood-compatible coating is polyethylene glycol, methoxy polyethylene glycol or mixtures thereof having a molecular weight of between about 100 and 20,000 grams per gram mole. The blood-compatible coating is applied as an anti-blocking agent. See column 2 lines 18 to 37.
The present inventors have found a coating for medical devices which avoids the aforementioned problems comprising a first coating of a hydrogel polymeric substance, and a second coating of a hydrophobic silicon having a hydrophilic surfactant which can impede blocking or sticking of two surfaces for improved lubricity, as well as the shelf life.